Plastic lenses are lightweight, highly tough and easy to be dyed. Properties particularly required for plastic lenses are: low specific gravity; high transparency; low yellowness; high refractive index and high Abbe number as optical properties; high heat resistance; high strength; and the like. A high refractive index allows a lens to be thinner, and a high Abbe number reduces the chromatic aberration of a lens.
Recently, many organic compounds having a sulfur atom have been reported as raw materials of plastic lenses for eyeglasses. In particular, polythiol compounds having a sulfur atom are known to be useful compounds, and for example, such polythiol compounds are reacted with isocyanate to be used as polythiourethane resin having excellent impact resistance, or are reacted with episulfide to be used as resin having an excellent refractive index (Patent Documents 1 and 2). However, polythiol compounds are easily colored by long-term preservation or exposure to high temperature during transportation or the like, and it causes the problem of color phase deterioration of optical materials obtained by polymerizing the polythiol compounds.